In order for the transaction to be final, i.e., not reversible, the transaction needs to be included in a block. The
nLockTime parameter can be used to guarantee that the transaction cannot be mined prior to a certain block height or a certain unix time stamps. In other words, the protocol guarantees that a specific transaction cannot be finalized prior to a certain time.
Imagine that you deposit 1 BTC into a 2/2 multisignature account and you intend to use this to make incremental payments to some other party. One of the keys in the multisig account is held by you and another one is held by the other party. Each time you want to make a payment to the other party, you sign a transaction from the multisig to the other party and give the signature to them. The other party can then, at any time, choose to publish the transaction you just signed by adding their own signature and handing the transaction to miners, or they can wait to see if any more payments (i.e. signatures) come from you.
Before depositing the bitcoins to the multisig address, the other party should send you a signature of a transaction with
nLockTime set to 1000 blocks in the future. This transaction transfers the entire deposit of 1 BTC back to you. If they do that, then you can always get your deposit back (after 1000 blocks) even if the other party for some reason dissapears or chooses to act destructively.
nLockTime makes it possible to create trust-free payment channels.